1. Field of the Invention
This invention relates to a method and apparatus for tuning the modal response of a decoupled micro-machined gyroscope. In a decoupled gyro, there are multiple vibration modes that are instrumental in the gyro performance. To reach the maximum performance of the gyro it is necessary to independently tune, or set the frequency of, these vibration modes.
2. Description of the Prior Art
This invention relates to sensing devices which utilize the gyroscopic principle, i.e., measuring the Coriolis force created by the conservation of momentum of a moving body. Specifically, the invention concerns devices called micro-gyros, which are small and inexpensive. They rely on conservation of momentum of a structure having limited oscillation motion, rather than full rotation. They are able to withstand rough environments for long periods of time.
In this field, the terms used to describe the directions of motions and of forces can be confusing. Applicant in describing and claiming the present invention will refer to the three separate directions (which are orthogonally related to one another) as follows: (a) the driven element, which is cause to oscillate at a predetermined, arbitrary rate inside the gyro, moves around the drive axis; (b) the velocity of the gyro environment, which is to be determined by the gyro, is around the rate axis; and (c) the Coriolis force, which is a function of the velocity of the gyro environment, is measured by motion of a sensing element around the output axis.
A number of patents have been issued to the Charles Draper Laboratory for such micro-gyro sensors, including U.S. Pat. Nos. 5,016,072; 5,203,208; 5,349,855; 5,408,877; 5,535,902; and 5,555,765. The earliest of the listed Draper patents refers to xe2x80x9cU.S. Pat. No. 4,598,585 to Boxenhorn, which discloses a planar micro-mechanical vibratory gyroscope adapted for small geometry configurations which may be constructed using semiconductor fabrication mass production techniquesxe2x80x9d.
In Draper U.S. Pat. No. 5,016,072, a single element mass is supported by a system of flexible linkages, made of semiconductor material, to allow for movement in two axes. A system of electrodes drives the mass to vibrate in one axis, and senses the motion of the mass due to Coriolis force in another axis. In another patent issued to Draper, U.S. Pat. No. 5,203,208, the same concept is extended to a symmetrical support linkage system. Draper U.S. Pat. No. 5,349,855 is another micro gyro design wherein an element mass is supported by a system of flexures. The element is driven laterally, and reacts rotationally due to Coriolis force. U.S. Pat. No. 5,408,877 issued to Draper relies on moving a single proof mass along one linear axis, and senses the motion of the same element along an orthogonal axis due to Coriolis force. Draper U.S. Pat. No. 5,555,765 shows a micro gyro using a single mass element formed into the shape of a wheel. By oscillating the wheel mass, a rotation about an axis normal to the plane of the wheel will create Coriolis force that will tilt the wheel.
U.S. Pat. No. 5,359,893 issued to Motorola uses a pair of elements supported in an xe2x80x9cH-shapedxe2x80x9d linkage frame so that angular velocity can be measured in two perpendicular axes. U.S. Pat. No. 5,488,862 of Neukermans et al involves a design with an outer torsional frame that is excited to tilt about an axis in the plane of the frame; an inner frame responds to the Coriolis force by oscillating (and carrying with it the outer frame) in an axis orthogonal to the outer frame axis. Both the drive and the sensing mechanisms rely on piezo-voltage actuators and sensors mounted on the hinges. In addition to the listed patents, substantial micro-gyro work has been done by the Berkeley Sensor and Actuator Center.
There are several significant defects in the prior art micro-gyros. With the exception of U.S. Pat. No. 5,488,862, reliance is on a single mass element for both driving and sensing functions. This coupling of the driving and output motion severely limits the sensitivity of the gyro. For example, as the single element is driven to vibrate, a key parameter that affects the driving mechanism is the alignment between the element and the drive electrodes. In the presence of an angular rate, the Coriolis force will create a secondary motion on the same element, thereby disturbing the alignment between the mass and the driving electrodes. Complex control schemes are necessary to compensate for such undesirable motions.
Furthermore, with only a single mass element, it is difficult to match the two resonant frequencies. Corrections are limited to support linkages only; any correction made to the mass element will alter both driving and output resonance simultaneously. Another complication is that the proximity of a single element to multiple electrodes leads to stray capacitance and coupled electric fields that are significant sources of electrical noise.
In U.S. Pat. No. 5,488,862, although two elements are used, the design does not allow for independent movement of each element. The outer frame is rigidly connected to the inner frame, so that the two frames essentially behave as a single mass element. When the inner frame rotates, the outer frame rotates with it. Another shortcoming in that design is that the outer frame has severely limited movement, due to typically very small thickness spaces (usually micro meters) in micromachining. The limited rotation of the frame results in low angular momentum, and hence low gyro sensitivity.
In all prior art designs there is also a lack of ability to electronically correct for imbalances of the system due to manufacturing tolerances. The problem is worsened by the use of a single element for drive and output motions. Since the same physical element is driven and sensed, due to mechanical coupling any forces used to balance the mass will often generate undesirable signals that corrupt the intended signal. For the same reason, self-testing of the micro gyro in prior art designs is very difficult.
In response to these limitations the assignee of the present application developed a multi-element gyro as disclosed in U.S. Pat. No. 5,955,668. The gyro disclosed there essentially separates the mass (momentum of inertia) of the constant motion element driven to oscillate around the drive axis from the mass (momentum A of inertia) of the variable motion sensing element which creates the measured force. This may be accomplished using: (a) an outer ring-shaped element which oscillates around the drive axis, and (b) an inner disk-shaped element which oscillates, or rocks, around the output axis as a result of the Coriolis effect. The torque around the output axis may be measured by any suitable means, such as capacitance, magnetic force, piezoelectric or piezoresitive effect, or optical signals. The dual-element configuration permits matching of the resonant frequency of the ring in its oscillation with the resonant frequency of the disk in its rocking. The dual-element structure also permits the ring and the disk to be excited independently, so that each can be dynamically compensated for manufacturing tolerances by counterbalancing. The sensing element, e.g., the disk, may be supported by a pair of hinges, or flexures, that permit tilting about an axis formed by these hinges. The hinges are connected to posts, or xe2x80x9canchorsxe2x80x9d which support the entire device on a substrate. The hinges provide mechanical isolation of the gyro from the stresses in the substrate. The design of the anchor hinges can be modified to accommodate either single or dual-axis sensing. The essential separation of the driven element from the sensing element may also be accomplished using outer and inner rectangular-shaped elements, which are caused to move, respectively, in linear directions along two orthogonal directions.
However, the nature of the flexures or hinges also affects what range of frequencies can be realized within the optimum performance ranges of the driven and sense masses, i.e. the disk and ring masses. Changing a design parameter in one changes in the ranges possible frequencies for the other despite the fact that they can be independently excited. Therefore, what is needed is some type of design wherein a decoupled gyro can be more freely designed so that a design parameter in one of the decoupled masses can be chosen without necessarily restricting the design parameters which can be chosen in the other one of the decoupled masses.
The invention is a method of setting the drive and sense frequencies of a gyroscope having a drive mass and a sense mass which are coupled together by a flexure assembly. The method comprises the steps of selecting a drive stiffness, Kd. Geometric parameters of the flexure assembly are selected to obtain a desired drive frequency, xcfx89d. At least one configurational parameter of the flexure assembly is selected to obtain a desired sense frequency, xcfx89s. The gyroscope is then evaluated to determine if it has obtained desired performance and size envelope characteristics. If it has not, then the steps of selecting a drive stiffness, Kd, selecting geometric parameters of the flexure assembly to obtain a desired drive frequency, xcfx89d, and selecting a configurational parameter of the flexure assembly to obtain a desired sense frequency, xcfx89s, are repeated until it is determined that the gyroscope has obtained desired performance and size envelope characteristics.
The step of selecting geometric parameters of the flexure assembly to obtain a desired drive frequency, xcfx89d, comprises the step or steps of selecting length and/or width of at least one individual flexure within the flexure assembly. In the illustrated embodiment the selection of length and width of at least one individual flexure within the flexure assembly comprises selecting length and/or width of each individual flexure within the flexure assembly. Also in the illustrated embodiment the step of selecting a configurational parameter of the flexure assembly to obtain a desired sense frequency, xcfx89s, comprises the step of selecting an orientation of at least one flexure within the flexure assembly relative to other ones of the flexures with the flexure assembly.
Again in the illustrated embodiment the individual flexures within the flexure assembly are oriented symmetrically about an axis of symmetry of the gyroscope. The step of selecting an orientation of at least one flexure within the flexure assembly relative to other ones of the flexures with the flexure assembly comprises the step of selecting one of a possible number of orientations of at least one flexure to the axis of symmetry of the gyroscope. In particular, the flexure assembly includes at least one pair of flexures. The step of selecting a configurational parameter of the flexure assembly to obtain a desired sense frequency, xcfx89s, comprises selecting an angle which the pair of flexures makes to each other. Still more specifically, in the illustrated embodiment the flexure assembly comprises two diametrically opposing pairs of flexures and where selecting an angle which the pair of flexures makes to each other comprises setting a dihedral angle between each of the flexures of the two diametrically opposing pairs.
In the illustrated design the step of selecting geometric parameters of the flexure assembly to obtain a desired drive frequency, xcfx89d, comprises selecting length, L, and width, w, of four flexures formed into two pairs comprising the flexure assembly, according to the equation       ω    d    2    =            4      ⁢      E      ⁢              xe2x80x83            ⁢              w        3            ⁢      t      ⁢              xe2x80x83            ⁢              R        2                    12      ⁢              L        3            ⁢              I        d            
where E is the Young""s modulus of the flexure. t is the process thickness of the flexure. Id is the rotational moment of inertia of the drive mass about a rate axis. and R is the radius of the drive mass where the drive mass is a ring-shaped mass.
In the same situation the step of selecting a configurational parameter of the flexure assembly to obtain a desired sense frequency, xcfx89s, comprises selecting xcex8 according to the equation       ω    s    2    =                    4        ⁢        E        ⁢                  xe2x80x83                ⁢        w        ⁢                  xe2x80x83                ⁢                  t          3                ⁢        sin        ⁢                  xe2x80x83                ⁢        θ        ⁢                  xe2x80x83                ⁢                  R          2                            12        ⁢                  L          3                ⁢                  I          s                      .  
The invention further includes the improved gyroscope which is designed by the above method.
While the method has been described for the sake of grammatical fluidity as steps, it is to be expressly understood that the claims are not to be construed as necessarily limited in any way by the construction of xe2x80x9cmeansxe2x80x9d or xe2x80x9cstepsxe2x80x9d limitations under 35 USC 112, but to be accorded the full scope of the meaning and equivalents of the definition provided by the claims whether by the judicial doctrine of equivalents or by statutory equivalents under 35 USC 112. The invention can be better visualized by turning now to the following drawings wherein like elements are referenced by like numerals.